As a typical main circuit construction for power converters, there has generally been used an indirect-type AC power converter in which commercial AC voltage is converted into DC voltage via a rectifier circuit and a smoothing circuit and then AC power is obtained by a voltage-type converter. Meanwhile, as a method for obtaining an AC output directly from AC voltage, there is known a three-phase capacitorless inverter which eliminates the need for large-scale capacitors or reactors for use of smoothing voltage pulsations due to commercial frequency so that downsizing of power converters becomes implementable.
As a conventional first power converter, there is provided a direct-type AC power conversion circuit in which sextuple-harmonic currents toward the power supply side are suppressed (see, e.g, JP 4488122 B).
As a conventional second power converter, there is provided one in which pulsating voltages are detected to modulate a voltage-type inverter so that voltage pulsations are compensated to obtain output voltages equivalent to those of conventional inverters (see, e.g., JP 61-48356 B).
In the above-shown conventional second power converter, when large harmonics of the motor slot are involved in the motor load, it is conceivable to increase the reactor capacity so that the resonance frequency of an LC filter is lowered to suppress power harmonics, in which case, however, features of the capacitorless method could not be utilized. As a result, with the conventional first power converter, in cases where harmonic components occur to high orders in the motor load, there arises a necessity for a plurality of control circuits and moreover, strictly, harmonic components cannot be erased without considering phase characteristics, which causes a problem of complicated control circuits.